1. Field of the Invention
The present invention relates to a photovoltaic element such as a solar cell or a sensor having a plurality of pin junctions for improving the conversion efficiency and covered with a protective member in consideration of long-term outdoor use.
2. Related Background Art
Various photovoltaic elements have already been put to use as independent power supplies for electric appliances and as an alternative to electric power systems. At the present time, however, their prices for a unit amount of generated power when they are used as an alternative to electric power systems are still high and studies and development of them are vigorously promoted.
Thin-film photovoltaic elements have attracted attention since structure sensitivity of amorphous silicon thin film such as that of crystalline silicon under doping by substitution was reported (see W. E. Spear, P. G. Lecomber “Solid State Commun.” Vol. 17, p. 1193, 1975). Studies of thin-film photovoltaic elements, e.g., the one described in U.S. Pat. No. 5,298,086 have been vigorously made.
In recent years, the photoelectric conversion efficiency of thin-film photovoltaic elements has reached 13%, as reported in J. Yang, A. Banerjee, S. Guha “Appl. Phys. Lett.” Vol. 70, No. 22, p. 2 and p. 2975, 1997.
Also, on microcrystalline silicon studied and developed in the past but left behind crystalline silicon and amorphous silicon in being put to practical use, studies have been vigorously made since it was reported that a satisfactorily high degree of photoelectric conversion efficiency can be obtained by using microcrystalline silicon, and that no photodegradation of microcrystalline silicon is observed (see J. Meier, P. Torres, R. Platz, H. Keppner, A. Shah “Mat. Res. Soc. Symp. Proc.” Vol. 420, p. 3, 1996).
Photovoltaic elements having a photoelectric conversion efficiency of 10.7% have recently been reported (see Japanese Patent Application Laid-Open No. H11-330520 and K. Yamamoto, A. Nakajima, Y. Tawada, M. Yoshimi, Y. Okamoto, S. Igari “Pro. Of 2nd World Con. Photovoltaic Energy Conversion” p. 1284, 1998, for example).
Schemes to further improve the efficiency by stacking a plurality of pairs of a pin junction mainly formed of an amorphous semiconductor and a pin junction mainly formed of a microcrystalline semiconductor have also been put forth (see Japanese Patent Application Laid-Open Nos. H11-243218 and H11-243219, for example).
In U.S. Pat. No. 5,298,086 is described an arrangement in which the photocurrent through a pin junction of the best characteristics is made the smallest of the photocurrents through a plurality of pin junctions.
In U.S. Pat. No. 6,153,823 is described an arrangement in which the photocurrent through a pin junction having a smaller degradation rate after the provision of a protective member is made smallest.
Further, in Japanese Patent Application Laid-Open No. H11-243219 is described a technique of making smallest the photocurrent generated at a pin junction comprising an i-type microcrystalline semiconductor in a photovoltaic element having a stack of a pin junction comprising an i-type amorphous semiconductor and a pin junction comprising an i-type microcrystalline semiconductor.
From these findings based on the studies heretofore made, it is easily conceivable that in the case of fabrication of a photovoltaic element in which a first pin junction comprising an i-type amorphous semiconductor, a second pin junction comprising an i-type microcrystalline semiconductor and a third pin junction comprising i-type microcrystalline semiconductor are formed in this order from the light incidence side, the second pin junction on the light incidence side of the second and third pin junctions may be ordinarily set thinner than the third pin junctions, low transportability of carriers excited by light may suffice, and the photocurrent generated at the second pin junction is made smallest.
Apart from these techniques, a technique of providing a layer functioning both as an antireflection layer and as an electrode on the light incidence side of a pin junction to effectively utilize light is also known.
There is a demand for designing a solar cell so that the solar cell is capable of long-term outdoor use. Light and flexible protective members other than glass protective members ordinarily used have also been proposed. An example of such protective members is one having a transparent fluoride polymer thin film such as a fluororesin film formed as an outermost surface layer and some of various thermoplastic, transparent, organic resins, e.g., an acrylic resin used as an encapsulant resin on the photovoltaic element side of the outermost layer (see Japanese Patent Application Laid-Open No. H08-139347). It is generally known that this kind of protective member can also have an antireflection effect if the refractive index of the protective member is set to a value between the refractive index of a transparent resistive layer and the refractive index of air.
While techniques such as those described above are being studied, it is most important, from the standpoint of use of photovoltaic elements, to determine whether the totality of the amount of material used, the cost of fabrication, the installation area, the external appearance, etc., of a photovoltaic element is commensurate with the generated energy obtained during a long period of ten to twenty years. That is, only the efficiency of conversion from light to electricity is not always the most important. This also explains the reason that thin film photovoltaic elements having the potential to be fabricated at a much lower price while being slightly inferior in conversion efficiency in comparison with crystalline photovoltaic elements having a high conversion efficiency attract attention.
Photovoltaic elements have been improved by the above-described techniques but their prices for a unit amount of generated power when they are used as an alternative to electric power systems are still high. There is a demand for a higher photoelectric conversion efficiency and for a method for fabrication at a low cost.